1. Field of the Invention
The present invention relates to an optical head for use in an optical recording/regenerating unit which optically records and regenerates data to/from a recording medium.
2. Description of Related Art
FIG. 1 shows construction and the light path of a conventional optical head disclosed in the Japanese Utility Model Application Laid-Open No. 61-68322 (1986). In FIG. 1, a beam (divergent beam) 1 emitted from a semiconductor laser (not shown) is deflected by a half mirror 2. In the traveling direction of the deflected beam is arranged a recording medium 4. Between this recording medium 4 and the half mirror 2 is arranged an objective lens 3 which focuses the deflected beam onto the recording medium 4. The beam reflected from the recording medium 4 is transmitted by the half mirror 2. In the traveling direction of the transmitted beam is arranged a photo detector with a light receiving area split into two parts 5a, 5b. This two-split photo detector is connected with an operational amplifier 6, which deducts an output of one part 5b of the photo detector from an output of the other part 5a of the photo detector.
With the above construction, the divergent beam 1 emitted from the semiconductor laser is deflected 90 degrees sideward by the half mirror 2 and projected onto the recording medium 4 by the objective lens 3. When this beam is focused on the recording medium 4, a small photo spot with a diameter of about 1 .mu.m is formed on the recording medium 4. The beam reflected from the recording medium 4 is transmitted by the objective lens 3 and half mirror 2, then projected on the two-split photo detector 5a, 5b. According to the distribution of the quantity of divergent beam 1 entering the two-split photo detector 5a, 5b, the operational amplifier 6 detects a tracking error of the small photo spot on the recording medium 4. In response to this tracking error signal, the objective lens 3 is driven in the direction shown by an arrow T in FIG. 1, and tracking is carried out.
On the other hand, a focusing error detecting optical system (not shown) detects a focusing error of a small photo spot on the recording medium 4. In response to the focusing error signal, the objective lens 3 is driven to the direction shown by an arrow F in FIG. 1, and focusing is carried out.
Now will be described below a manner of detecting the tracking error. In FIG. 2(a), (b), (c) and (d), fragmentary sectional views of the recording medium 4 are shown on the left and views of the partial optical path made by the optical head. As shown in FIG. 2(a) and (c), where the small photo spot is located either in the center of a guide groove 4a on the recording medium 4 or in the center in between the grooves 4a, both quantity of the beams entering respective parts of the two-split photo detector 5a, 5b are equal.
However, as shown in FIG. 2(b), when the small photo spot deviates in the direction of crossing the guide groove 4a, the beam is diffracted by the guide groove 4a, and the quantity of the beam entering one part 5b of the photo detector (hatching portion in FIG. 2) is less than that of the beam entering the other part 5a of the photo detector. On the contrary, as shown in FIG. 2(d), where the small photo spot deviates in the opposite direction to the above case shown in FIG. 2(b), the quantity of the beam entering one part 5a of the photo detector (hatching portion) is less than that of the beam entering the other part 5b of the photo detector. As a result, whether the small photo spot is located properly in relation to the guide groove 4a or not and which direction the small photo spot deviates are detected by the difference between the outputs from two parts 5a and 5b of the photo detector. This method for detecting a tracking error is called a push-pull method.
As shown by the broken line in FIG. 3, in such conventional optical head as mentioned above, where the objective lens 3 moves in the direction of tracking (direction shown by an arrow T in FIG. 1) during tracking, the beam entering the two-split photo detector 5a, 5b is shifted, then, there is a problem that an offset is generated in the tracking error signal, which adversely affects the tracking.